Electrochemical sensors are known, the measuring principle of which is based on ion diffusion. This ion diffusion takes place, using a suitable electrolyte, usually via a semi-permeable membrane, for example a glass for H+ and Teflon for CO2. Such electrochemical sensors enable primarily the H+-concentration to be measured and indirectly, via the measurement of the H+-concentration, also the concentration of, for example, CO2 or O2 to be determined. The measurement of the H+-concentration takes place with a pH electrode, also termed a glass electrode.
A modification of the pH electrode is the pO2 electrode, also termed a Clark electrode, which serves for the measurement of the O2 concentration.
A further modification of the pH electrode is the pCO2 electrode also termed a Severinghaus electrode which permits the measurement of the CO2 concentration. In the Severinghaus electrode the CO2 diffuses through a membrane (Teflon) into an electrolyte with an NaHCO3 solution with: CO2+H2O<->H2CO3<->H++HCO3 applying. The H+-concentration is measured with the pH electrode and the CO2 value derived therefrom.
Such electrochemical sensors are used amongst other things for the measurement of blood gas values such as the CO2 concentration or the O2 concentration in the blood. The electrochemical sensor is applied to a position of the human body with a good circulation of blood in order to measure the transcutaneous carbon dioxide partial pressure (tCpCO2) or the transcutaneous oxygen partial pressure (tcpO2). Extensive information concerning these generally known measurement methods are for example to be found in the following review article “Noninvasive Assessment of Blood Gases, State of the Art” by J. S. Clark et al., Am. Rev. Resp. Dis., Vol. 145, 1992, pp 220-232.
It is also known to provide the electrochemical sensor with additional sensors, for example with LEDs and photoelectric sensors in order to simultaneously carry out a pulsoximetric measurement. A combination sensor of this kind for the combined measurement of the oxygen saturation of the haemoglobin in arterial blood and also of the arterial carbon dioxide partial pressure is known from the document EP 0 267 978 A1. This combination sensor includes as an electrochemical sensor, a Severinghaus electrode for the measurement of the transcutaneous CO2 partial pressure and also an arrangement for the measurement of the oxygen saturation (SpO2) by means of pulsoximetry.
A disadvantage of electrochemical sensors is the fact that their preparation and maintenance is very demanding and requires trained specialist personal. For the Severinghaus electrode the semi-permeable membrane and the electrolyte which is located between the membrane and the sensor head must, for example, be regularly exchanged in order to ensure a problem-free operation. This servicing work is very demanding because the measurement accuracy is dependent on the thickness of electrolyte layer and also on the precise arrangement of the membrane. The reproducability of the measurement accuracy is of central importance, because vital parameters, such as the CO2 content in blood are measured with the electrochemical sensor. A faulty measurement of this parameter could prove lethal for a patient. The electrochemical sensor is, moreover, frequently used in a hectic environment, such as in an intensive care station in the hospital. The electrochemical sensor is however also increasingly frequently used in the home area (home care) in order to monitor patients at home.